Why did Allah permit marriage to four women, while He mentioned that men would not be able to be just among them, and that whoever fears injustice should restrict himself to one? And does this mean that marriage to more than one is undesirable?

The justice that is obligatory between wives is in lodging and maintenance. This is what Allah permitted polygyny for those who are able to fulfill it. However, it differs from the inclination and affection of the heart, which the husband does not control. Allah Almighty said: "And you will never be able to do perfect justice between wives even if you eagerly desire it. So do not incline completely, leaving her as if suspended." (An-Nisa: 129).